1. Field of the Invention
The present invention relates to a method of coating a copper film on a ceramic substrate, and particularly to a method of providing an improved adhesion between the copper film and the ceramic substrate.
2. Disclosure of the Prior Art
A method of coating a copper film on a ceramic substrate is provided in Japanese Patent Publication KOKOKU! No. 63-4336. That is, the method uses a paste comprising at least one selected from the group consisting of compounds of copper, zinc, cadmium, bismuth, and lead. The paste is applied to a surface of a ceramic substrate to form an undercoat. After the ceramic substrate having the undercoat is heated at a temperature between 350.degree. C. and 900.degree. C. in a non-oxidative atmosphere to precipitate metal or alloy particles from the undercoat, it is treated by a solution including palladium and/or platinum ions to substitute surface portions of the metal or alloy particles by palladium and/or platinum. Subsequently, a metallic film of nickel, cobalt or copper, is formed on the treated undercoat by electroless plating. However, this prior art shows that a maximum adhesion strength of the copper film is only 2.75 kg/5 mm.phi., that is, 0.56 kg/4 mm.sup.2.
On the other hand, Japanese Patent Publication KOKOKU ! No. 3-69191 discloses that a copper undercoat having a thickness between 0.5 .mu.m and 2 .mu.m is formed on a substrate of aluminum oxide by electroless plating. The undercoated substrate is heated at a temperature between 300.degree. C. and 900.degree. C. in an oxidative atmosphere, and then heated at a temperature between 200.degree. C. and 900.degree. C. in a reducing atmosphere. After a copper film having a thickness between 0.5 .mu.m and 2 .mu.m is formed on the reduced undercoat by electroless plating, an additional copper film is formed on the copper film by electroplating to obtain a required thickness thereof. However, in this method, since the oxidized undercoat must be reduced at the high temperature range, there causes a problem that the reduced undercoat usually exhibits a poor wetting property to a plating solution used for the electroless plating. This will bring about a poor adhesion between the copper film and the reduced undercoat. In addition, there is a problem that an expensive furnace for controlling the reducing atmosphere at the high temperature range is needed in this method.
Thus, there is room for further improvement with regard to the adhesion between the copper film and the ceramic substrate.